Laboratory study of fire geometry and plume structure

The geometry of an active wildfire determines some aspects of fire behavior, as well as potentially determining the strength, behavior, and chemical concentrations within the fire's updraft. The mixing of air inside and outside of the plume affects plume temperature and moisture, thereby exerting a strong and nonlinear influence on overall plume height and updraft velocity.  Terrain, fuels and horizontal wind can each exert a strong influence on fire geometry and thus on these other characteristics.  Most of the plume models used for wildfire smoke modeling assume either point sources or virtual point sources below the ground to yield a circular area source at the ground. We use a laboratory water tunnel to study the effects of fire geometry on mixing and plume length.  Negatively buoyant salt water with a visible tracer is released through an opening atop the tunnel and the plume structure and evolution are visually documented.  We vary the geometry of the opening, including a single circular area, circular arcs of equal area, and combinations of arcs and lines, while constraining the total buoyancy flux and tunnel flow speed.  Plume characteristics for each configuration are observed and compared, and implications for fire behavior and smoke transport are discussed.